The effective filament size of a Nb3Sn conductor in a high field magnet is an important parameter in reducing the magnetization hysteresis losses while charging and discharging the magnet and increasing the stability of said magnet. In a “low” current internal tin conductor (as used in fusion magnets), the individual filaments of a multifilamentary, multiple restacked billet are separated by copper far enough from each other that they do not bridge after reaction (reaction of Nb alloy filaments to A15 results in a volume expansion of ˜30%), and the effective filament dimension is the size of the individual filaments. In a “high” current internal tin conductor, copper is minimized and so all the Nb filaments grow into each other, merging, and forming large rings of superconductor. In this case, the effective filament diameter (deff) is equal to that of the entire subelement instead of the individual filament. As will be further evident in the following, the term “subelement” refers to the subassembly which when tightly grouped (packed) with like subassemblies forms a precursor assembly for the final superconductor wire.
Some applications require both high current and small effective filament diameters. To date, there have been several basic approaches to decreasing the filament sizes for high current internal tin conductors. The first is to decrease the subelement size by either packing more subelements in a restack (lessening the ratio of the subelement size to wire diameter size) or drawing to a small wire diameter. The drawbacks of these approaches are as follows:    (1) Fabrication of a restack with a large number of small size subelements is difficult to accomplish. More subelements results in more surface area to keep clean and free of inclusions. This negatively affects wire piece length and yield.    (2) The use of wire drawn to a smaller size is only feasible if the end use application of the wire is flexible in that regard. Most of the time, an application (magnet design) requires wire of a specific strand diameter and hence the subelement size needs to be achieved for that diameter.    (3) The large amount of strain required to draw the individual subelements small can cause the filament components to work harden and thin to such an extent that they fail, resulting in wire breaks and poor piece length and yield.
A further prior art approach is to subdivide a subelement by copper spacers or bent plates of Ta40% Nb sheets. The problems with these prior art divider techniques are as follows:    (1) Copper spacers tend to deform to such an extent that the reacted sections bridge the copper separated section. In addition, the inclusion of these excess copper channels lowers the relative Sn to Cu ratio, which hinders A15 reaction and results in a lower current density. The ˜30% volume expansion of Nb upon conversion to Nb3Sn also cause the filaments to grow across the copper spacers and merge.    (2) Ta40% Nb sheets, while not highly reactive, do react enough to form an A15 superconducting layer that causes some bridging problems.    (3) Ta40% Nb sheets prevent diffusion of the Sn across them, which inhibits uniform distribution of tin throughout the wire cross-section, resulting in non-optimal filament reaction and lower Jc.    (4) The use of sheets (e.g., Ta40% Nb) can cause yield/wire breakage problems because sheets are difficult to clean and inspect for flaws.    (5) Inserting tantalum, Ta40% Nb, or Cu sheets in the midst of a stack is difficult in the manufacturing process, disrupting the orderly stacking of the hexes.